1. Technical Field of the Invention
The present invention relates to radiation sources. More particularly, and not by way of any limitation, the present invention is directed to a system and method for providing coherent and substantially uniform radiation or illumination for applications such as, for example, emission microscopy, medical instrumentation and nuclear technology.
2. Description of Related Art
Illumination or radiation sources are important in many industrial, scientific and medical applications. While some of these applications may permit the use of conventional sources, several applications require concentrated and highly bright but uniform illumination or radiation. For example, laser devices which produce intense monochromatic illumination have been used in the past for applications with such special illumination requirements. However, current use of laser illumination is beset with several shortcomings and deficiencies, as will be described in reference to two exemplary applications set forth below.
Light (or photon) emission microscopy is a common failure analysis technique used for analyzing semiconductor integrated circuit (IC) devices. The considerations involved in using photon emission to successfully analyze defects and failure mechanisms in CMOS ICs are well known. IC failure analysis using an emission microscope is performed by collecting visible (390-770 nm), and sometimes near infrared (NIR) (770-1000 nm, with the typical IR band defined as 770-1500 nm), wavelength photons emitted from transistors, p/n junctions, and other photon-generating structures on or near the top (front), electrically-active, silicon surface. These photons are transmitted through the overlying, relatively transparent dielectric layers, passing between or scattered around the patterned, opaque metal interconnections. Detection of photons that emerge from around these overlying layers is referred to as frontside light emission analysis. Correspondingly, imaging light passing through the silicon substrate and emerging from the bottom (back) is referred to as backside light emission analysis.
Custom and commercial systems are routinely used for light emission analysis. The spectral characteristics for these systems are usually dependent upon the type of detector chosen. Most commercial systems use detectors based on image intensifiers or CCD arrays. Although current systems can provide detectors with extended NIR capability for backside analysis, most systems have very low response to photons with wavelengths beyond 1 .mu.m.
There is an increasing interest in backside light emission analysis. This is driven primarily by the advancement of IC fabrication technologies with additional opaque conductor layers and packaging technologies that typically obscure the active side of the die. Backside analysis takes advantage of silicon's transmission of photons with energies less than its indirect silicon bandgap energy, corresponding to wavelengths greater than around 1.107 .mu.m (for undoped silicon). It is commonly known that silicon becomes less transparent as dopants are added. Because of this phenomenon, the heavily doped substrates often used with newer technologies will attenuate NIR light emitted from the active circuits. These and other factors are stimulating research for solutions, including improved substrate thinning techniques, increased NIR imaging sensitivity, and spectral analysis.
It is well known that different types of photon emission processes can be distinguished by their spectra. Photon emission from defects or abnormal operation of silicon microelectronic devices generally falls into the following categories: forward or reverse biased p/n junctions, transistors in saturation, latchup, and gate oxide breakdown. While radiative recombination emission from silicon structures is generally centered around 1.1 .mu.m, commonly used cameras have spectral response centered in the 400-900 nm range and can thus capture only a small portion of the emitted light.
Traditional methods of NIR imaging use an optical filter in conjunction with a broad-spectrum illuminator such as a quartz halogen bulb. The desired wavelengths pass through the filter and are used in the microscope illuminating path. The desired wavelength is selected by the filter when the unwanted light frequencies are rejected. One of the problems of the current technologies is that when a more intense illumination source is used to address at least in part the issue of the poor quantum efficiency of backside emission, the optical filters get degraded or destroyed quickly due to heating. The problem is further compounded by the fact that as the filter bandwidth is narrowed, the total energy is also reduced from the source output. On the other hand, employing longer integration times, by taking the emitted light inputs over a considerable period of time, negatively impacts the through-put. Due to these constraints, it can be appreciated that the current illumination technology cannot provide intense, narrow bandwidth illumination that is highly advantageous in backside emission analysis.
Laser sources can provide very intense, substantially monochromatic illumination. When these sources are used in backside emission analysis, however, interference phenomena cause what is commonly known laser "speckle" that blur the illuminated image. The speckle is seen at least in part due to the nonuniform distribution of radiation energy, giving rise to "hot spots" and "dark areas". While techniques such as diffusing the laser light using a frosted glass, dithering (i.e., scanning the laser beam), et cetera, are sometimes used, they have not been sufficiently effective in alleviating the speckle problem in backside emission imaging. Further, it may be appreciated that the recent popularity of flipchip technologies, rapid escalation in the number of metal interconnect layers and advanced packaging techniques (for example, ball grid arrays, land grid arrays, etc.)--all of which obscure the front side view of the active area--make the need to solve the speckle problem more acute.
Laser illumination is also used in various medical diagnostics and treatment applications. For example, a new method of treatment for cancer in humans, known as "photodynamic therapy" (PDT), involves laser illumination. A mixture of chemicals known as "hematoporphyrin derivative" (H.sub.p D) is known to preferentially remain in cancer cells. When illuminated with light of a specific wavelength and in sufficient intensity, H.sub.p D undergoes a photochemical reaction and kills the cells in which it resides. It can be readily appreciated that PDT devices that would be advantageous to physicians in treating cancer must be capable of not only carrying intense radiation without overheating and self-destruction, but also of providing a uniform pattern of illumination so the physician can irradiate the entire treatment area with intense radiation lethal to the cancel cells, without leaving "dark areas" that could correspond or give rise to undestroyed cells.
For cancers occurring in tubular regions of the human body, the appropriate pattern of radiation for treatment is a uniform cylindrical pattern. Thus, for PDT treatment of esophageal cancer, an optical fiber is typically required to be with an apparatus at one tip that disperses light in a uniform cylindrical pattern. This optical radiating apparatus must produce a reasonably uniform pattern of intense light, without developing "hot spots", optical, thermal or mechanical damage.
One of the current solutions in this area involves providing a tip at an optical fiber that is subjected to chemical etching and roughening procedures. While scattered or diffused light is obtained from the tip, it does not possess the requisite cylindrical and/or circumferential uniformity in intensity. Another approach provides a fiber with a light carrying core that is tapered to a point, allowing the propagating light to escape at each point along the tapered core. Yet another solution involves coating the tip of an optical fiber. It is anticipated, however, that these solutions will add substantially to the cost and complexity of manufacture.